Preventing Invalid Characters in TextBox

[noorez]

In the TextBox control, I am trying to handle to the TextChanging event and check for invalid characters (only a custom set of characters are allowed). If any are found, I tried to set the event's Handled property to true to prevent the character from being rendered. However, this did not work; The character was still rendered.

Does the Handled property not work in the same way it does for WPF?

[stevemonaco]

You should use some preview events here. You can wrap them into a reusable attached behavior if you want. The following will block any numeric digits from entering the TextBox. You should handle both TextInput and the paste event, IMO. The paste handler was directly adapted from the TextBox.Paste implementation

<TextBox x:Name="box" />

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        box.AddHandler(TextInputEvent, Box_TextInput, Avalonia.Interactivity.RoutingStrategies.Tunnel);
        box.PastingFromClipboard += Box_PastingFromClipboard;
    }

    private async void Box_PastingFromClipboard(object? sender, Avalonia.Interactivity.RoutedEventArgs e)
    {
        if (GetTopLevel(this)?.Clipboard is not { } clipboard)
            return;

        e.Handled = true; // Never allow the default handler to handle pastes

        try
        {
            var text = await clipboard.GetTextAsync();
            if (!text.Any(char.IsDigit))
                box.Text += text;
        }
        catch (TimeoutException)
        {
            // Silently ignore.
        }
    }

    private void Box_TextInput(object? sender, Avalonia.Input.TextInputEventArgs e)
    {
        if (e.Text!.Any(char.IsDigit))
        {
            e.Handled = true;
        }
    }
}